1. Field of the Invention
This invention relates to a method of processing video signals for eliminating vertical picture fluctuations which appear upon the monitoring of video signals by a television receiver, such signals being reproduced by a video tape recorder (which will simply be called VTR hereinafter) at a tape speed different from that used for recording, so as to obtain stable reproduction pictures.
2. Description of the Prior Art
Generally in helical scanning type VTR's using rotary heads, the slopes of the traces of the scanning rotary heads upon reproduction are different from the slopes of the tracks scanningly recorded by the rotary heads, when the reproduction is a slow motion reproduction, a still picture reproduction or quick motion reproduction which is performed at a tape transportation speed different from that used during recording. For example, in a VTR wherein the tape scanning direction of the rotary head is the same as that of the tape transportation, the slopes of the scanning traces of the rotary heads are not steep in the case of slow motion reproduction and still picture reproduction, and are less steep than the recorded tracks. On the contrary, the slopes of the scanning traces upon quick motion reproduction are steep, and are steeper than the recorded tracks. Therefore, upon reproductions in these cases, the heads scan plural recorded tracks in one field of a video signal. So, not only noise bands are produced thereby, but also the number of the horizontal scanning periods (H number) reproduced in each scanning period of the heads becomes different from the H number used during recording. For example, in the case of still picture reproduction by a VTR for azimuth recording which performs an H number adjustment of 1.5H, the information corresponding to 264H are reproduced in one scanning period of the heads. (Usually this should be 262.5H).
In a VTR for azimuth recording, output signals cannot be obtained upon reproduction unless the heads used in recording are identical to those used for reproduction. So, since the H number for reproduction is different from that for recording as mentioned above, the time periods between adjacent vertical synchronous signals in the reproduced video signals become different from those in the recorded video signals. In the above-mentioned example, a time difference of 3H occurs between the adjacent fields. That is, the vertical synchronous signals cycle by 262.5H, 265.5H, 262.5H, 265.5H, . . . , namely by alternating long and short fields. In the slow motion reproduction also, a similar variation of the vertical synchronous signal cycle period occurs. But in this case, such long and short fields do not cycle by adjacent fields in contrast to the case of still picture reproduction. For example, in a 1/5 slow motion reproduction, 5 fields form one cycle. FIGS. 1a and 1b show models of representation cycles of the reproduced vertical synchronous signals during still picture reproduction and 1/5 slow motion reproduction, respectively.
The foregoing descriptions are directed to the case where each rotary head scans plural recorded tracks in each scanning period during reproduction. However, the above described variation of the vertical synchronous signal cycle period occurs quite similarly in the case also when the VTR employs an electric-to-mechanical conversion element for moving each rotary head in a direction perpendicular to the scanning direction thereof so as to perform complete on-tracking, because even in such a case, the H number reproduced in each head scanning period is different from that recorded therein.
If a composite video signal having such variations in vertical synchronous signal cycle periods in applied to a monitoring television receiver, the reproduced picture frames appear to be periodically shifted vertically on the television screen, which is the so called vertical picture fluctuation, resulting in extremely poor pictures, for the following reasons.
FIG. 2 is a block diagram of a main portion of a general television receiver for explaining the relationship between the video signal system and the vertical synchronous signal system thereof. Referring to FIG. 2, a composite video signal containing a time-base fluctuation component .PHI..sub.i (t) caused by the variation of the vertical synchronous signal cycle period (i.e. having different time periods between adjacent vertical synchronous signals) is applied to an input terminal 1 from a VTR. This illustrated portion of the television receiver is divided into a video signal system and a vertical synchronous signal system. The video signal component of the composite video signal is applied to the cathode or the control grid of a cathode ray tube 3 via a video signal amplifier 2. In this case, the time-base fluctuation component .PHI..sub.i (t) is transferred with change to the cathode ray tube 3.
On the other hand, in the vertical synchronous signal system, the vertical synchronous signal component is separated by a vertical synchronous signal separator 4, and is applied to a vertical deflection circuit 7 composed of a vertical deflection signal oscillator 5 and a vertical deflection signal amplifier 6. A blocking oscillator, for example, is used for the vertical deflection signal oscillator 5, and is usually triggered directly by the vertical synchronous signals for producing sawtooth wave signals corresponding to the vertical synchronous signal cycles. This sawtooth wave is amplified by the vertical deflection signal amplifier 6 composed of a vertical deflection driving circuit comprising means for compensating the display linearity and means for adjusting the amplitude of the deflection, and an output circuit. The thus amplified sawtooth is applied to a vertical deflection coil 8 of the cathode ray tube 3 for vertically scanning the electron beam to be applied to the phosphor screen of the cathode ray tube 3. Here, since capacitance coupling and/or transformer coupling are provided between the above described various means in the vertical deflection signal amplifier 6, this amplifier can be considered to have high pass transfer characteristics which cut off d.c. components. Since amplifier 6 also has a limit in its response to very high frequency signals, the vertical deflection circuit 7 can generally be regarded as having band pass transfer characteristics. So, when the input time-base fluctuation component .PHI..sub.i (t) passes through the vertical deflection circuit 7, the time-base fluctuation component is not unchangedly transferred therethrough, but is subjected therein to the time-base processing due to the above described frequency dependent variations in the response of the vertical deflection circuit. Thus, the time-base fluctuation component appears in the vertical deflection coil 8 in the form of .PHI..sub.o (t). As apparent from these, the video signals having the above time-base fluctuation component .PHI..sub.i (t) and the vertical synchronous signals having the time-base fluctuation component .PHI..sub.o (t) produced by the distorting of the time-base fluctuation components by the vertical deflection circuit 7 are mixed at the cathode ray tube 3. So, in the reproduced picture signals, a time difference between the above two time-base fluctuation components occurs, which causes vertical picture fluctuation.